In the Official Handbook of the Marvel Universe A-Z Hardcover #7: MAGIC entry, there was mention of numerous powerful mystics attending the Assemblage of Avatars in Atlantis around 19,000 BC to find a solution for the current conflicts between the Elder Gods. This is the one OHOTMU I do not have. Would anyone have a complete list of all the attendees and is this based upon a specific comic or from the OHOTMU only?

FWIW, I happened across a number of entries regarding the Assemblage on the Unofficial Appendix site... the only direct reference they give is OHOTMU Hardcover #7.

Here's the list of the entries I'd found (not sure if it's a complete list of the attendees, asterisks denote if the character has an MCP listing):
Linnea the Elfqueen
Teyanoga
Malgato
Zhered-Na*
Sorrow*
C'Thunda (in spirit form)
Tuzun Thune
Thulsa Doom*
Varnae*

Tuzun Thune has only appeared in the Conan and Kull comics, C'Thunda created the Chimera seen in Tomb of Dracula, Malgato was seen in the first couple issues of the second Ka-Zar series, Teyanoga's another Kull villain, and Linnea has only one major appearance of note (namely A 212).

"The official title of "Sorcerer Supreme" was not used until roughly 15,000 BC, when Agamotto chose his successor, but Agamotto became the paradigm that all future Sorcerer Supremes would aspire towards. He used his remarkable talismans, the Three Eyes of Agamotto to battle against demonic forces. He wrote the Book of the Vishanti so humans could learn magic and become enlightened. Mankind did prosper, and Agamotto helped the early sorcerers, alchemists and scientists to develop the great city Atlantis. Around 19,000 BC when conflicts broke out again between the Elder Gods, he summoned their representatives to Atlantis for the legendary Assemblage of Avatars. The summit was attended by all the greatest sorcerers of the ancient era, including Zhered-Na with her apprentice Dakimh, Lady Linnea, Retath, Sorrow, Teyanoga, Tuzun Thune, Akoos the Red Wizard, Thulsa Doom, Lord Varnae (still human at the time), and the spirit of C'Thunda, but very little was accomplished and soon after this Agamotto began to express frustration with mankind's limitations. Impressed with Zhered-Na, Agamotto continued to observe and assist her, and began grooming her to become his successor as humanity's protector."